In case of high performance structures, such as those intended for Aircrafts and Aerospace vehicles, the issue of Fastener Hole Quality is of great importance. Great care is required to prepare holes with precise dimensions and proper surface finish such that the structure (or work piece) can properly engage with the fastener. This proper preparation is intended to keep the hole and surrounding structure free from flaws such as Scratches, Undercuts, Burrs and other forms of discontinuities. In the case of Composite structures the issue of good hole quality is even more significant and harder to achieve.
Hole size and quality has a significant effect on fatigue life of the structural joints. Examination of fatigue type failures of aircraft structures indicate that fatigue failures are often initiated in regions where the structure interfaces with fasteners. It has also been shown that a proper amount of compressive residual stress induced by radial interference of an installed fastener improves structural fatigue life by resisting crack formation at the hole. It is therefore very important to employ procedures that prepare quality holes, and select fasteners with specific features for proper joint design, such that the structural joint can resist dynamic loading and enhance the fatigue life of the structure.
During the process of initial hole preparation, errors in manufacturing may cause a hole to be drilled oversized, non-symmetrical, or with surface flaws. Also, during the service life of the structure, it is often necessary to remove and replace installed fasteners to repair a joint, or individual fastener, showing signs of damage. During the fastener removal process the hole is often damaged. In both of these cases the standard size hole is often drilled to a larger diameter that can accommodate a non-standard oversized fastener. The economic impact of this is significant as oversized fasteners are expensive and often not readily available.
Another acceptable practice is to attach a thin wall cylindrical insert in the hole, most often with a bonding agent, and then install the fastener within this insert. The challenges involved with this approach are that of dimensioning the insert, the fastener, and the hole such that an intimate contact of adequate interface strength is kept within all the components. Due to dimensioning and manufacturing tolerances of this type of solution, relative movement of components within the structure may occur, leading to a reduction in structural fatigue life.
Consequently, a new fastener design, which alleviates the problems described above and allows for insertion into an oversized or reworked/reconditioned aperture in a structure or work piece, is needed.